Question about con trials

[Warren Walker]

Sitting on my deck this morning having a cup of coffee, I saw many airliners off in the distance leaving con trails.

My question is, what is the most contributing factor as to how long they will be.

Will a heavily laden cargo 747 using more fuel than a passenger 747 leave a longer trail, or does it have more to

do with air temp?

Also what effect does a head wind or tail wind have.

W.W.

[RC Storick]

They are not con tails. Chem trails and I know some will say I am crazy but look it up how many National weather service people have been let go for talking about it.

Here is one page http://topinfopost.com/2014/09/17/the-united-nations-exposes-chemtrails-100-proof-we-are-being-poisoned

[Motorman]

ROLMAO! 

[Gerald Arana]

They are not con tails. Chem trails and I know some will say I am crazy but look it up how many National weather service people have been let go for talking about it.

Here is one page http://topinfopost.com/2014/09/17/the-united-nations-exposes-chemtrails-100-proof-we-are-being-poisoned

Phtttttt! To you Roselind. AFAIC this is a huge pile of horse dung.  

In case she doesn't know it CA is in a massive drought! That's why the trees are dying...........there's no water to absorb. (or wasn't before the latest storms)
My hedge was just about gone (I watered it just enough to keep it alive) but now it is looking (almost) normal again.

There's an old saying "Don't believe anything you hear and only half of what you see".

That's more true now days than ever before AFAIC.

Politics suck, Jerry

[RC Storick]

If you make it rain in the ocean before it reaches land what happens? Drought  Everyone can make their own mind up but this is climate manipulation as I see it. If you oppose my theory that's fine and your right. I remember in the 60Ty's when sonic booms occurred and you would see con trails and they would disappear in a 1/2 hour or so, now they stay around all day. At the very least if they changed the fuel formula and it causes this ,it surely cant be good for the environment.

[BillLee]

ROLMAO! 

+1

[RC Storick]

Can someone explain these images? Don't say hoax as there are hundreds of them all you have to do is look for them.

[Howard Rush]

I can probably explain the first three.  People use water ballast tanks to do flight testing at different weights and CGs.  It's a quick way to change weight or CG.  I tried to zoom in on the people to see if there was anybody I recognized.  Resolution wasn't good enough.

[RC Storick]

I can probably explain the first three.  People use water ballast tanks to do flight testing at different weights and CGs.  It's a quick way to change weight or CG.  I tried to zoom in on the people to see if there was anybody I recognized.  Resolution wasn't good enough.

Makes sense to me thanks Howard. Now if we could just find out whats spraying out that pipe. Many photos of that sort of thing too. What ever it is it cant be good.

[john ohnimus]

Emergency fuel dump. Could be a flight test deal. We op's check them on the ground, but for certification purposes a manufacturer would have to do flight testing. And I'm 100% certain Howard is correct about the tanks, notice how they are oriented where the passengers sit. If they were for chemicals, they could put ALOT more in aux tanks in the cargo compartments below

[Jim Carter]

I'm definitely not one for "conspiracy theorys" but I did ask a question once to a group of highly educated and supposedly knowledgeable folk from which I've never received a plausible answer.  It's almost like there's no one who knows or I at least I haven't found anyone who can direct me to the person who can/could answer.  The question relates to the mid-stratosphere (11-30 miles), specifically, it is published that the air in the stratosphere is far thinner and more rarified than the troposphere where we and our weather exist.  Given the data that shows that 90% of the ozone layer lies in the 12-20.  So, since the advent of high altitude jet travel, military flights and so forth and considering it's been over 60 years for nearly every "civilized" nation on earth, isn't that clean rarified air becoming thinner and dirtier thus affecting the lower earth atmosphere, where we live, in an adverse way??  IF so, doesn't it seem that the amount of high altitude jet flights should be sharply curtailed before the damage is irreversible??  Yes, I know there are commercial interests involved as are military requirements, but as an ordinary nobody, I'm just curious.

I've read: "A typical commercial jet engine takes in 1.2 tons of air per second during takeoff—in other words, it could empty the air in a squash court in less than a second." Also, "At takeoff power, the B757 is limited to about 877C. It never really gets that high, but it does get real hot. If you are a Fahrenheit sort of guy, 877C translates to about 1611F."  Considering the consumption of "clean air" and the heating of the surrounding air plus the accidental or intentional waste of fuel and residues ..... is there a problem??  Jus' askin' ??

[RC Storick]

Emergency fuel dump. Could be a flight test deal. We op's check them on the ground, but for certification purposes a manufacturer would have to do flight testing. And I'm 100% certain Howard is correct about the tanks, notice how they are oriented where the passengers sit. If they were for chemicals, they could put ALOT more in aux tanks in the cargo compartments below

Like these?

 

This is probibaly BS

[Steve Fitton]

Those are for deicing tests, among other things.

Sparky I think you need some higher quality tinfoil for your hat.  Maybe Howard has some aerospace grade floating around.

[RC Storick]

Those are for deicing tests, among other things.

Sparky I think you need some higher quality tinfoil for your hat.  Maybe Howard has some aerospace grade floating around.

Well if I am wrong why do the CON trails hang around all day now and they use to disappear in a 1/2 hour or so. I can see this with my own eyes and don't need a hat for that but of coarse I am not sleeping.

[Eric Viglione]

Well if I am wrong why do the CON trails hang around all day now and they use to disappear in a 1/2 hour or so. I can see this with my own eyes and don't need a hat for that but of coarse I am not sleeping.

Contrails are nice...I like looking at them myself... if they are lasting longer than you remember from the old days, it's probably because todays air is damper. Maybe it is due to high carbon footprints and more upper atmosphere moisture... or because even though many stopped using aerosol deodorant, (coincidentally deodorant that had aluminum... which may have had a form of protection similar to tin foil hats) there is still a growing planet wide carbon output due to large volcanic output, repopulation of endangered gassy species, industrialization of places like China, India & Vietnam, etc. So the moral here, is that if you want to smell nice, and not be bothered by conspiracies, use aerosol deodorant as both rightguard, leftguard, and headguard!

EricV

[Howard Rush]

My question is, what is the most contributing factor as to how long they will be.

I don't know, but I can cipher part of it.

Will a heavily laden cargo 747 using more fuel than a passenger 747 leave a longer trail, or does it have more to do with air temp?

Cargo and passenger versions operate at about the same gross weights, so which version shouldn't matter.  I figure that the amount of water vapor that an airplane in cruise leaves behind per foot traveled is proportional to the square root of its gross weight.  Its cruise speed is also proportional to the square root of gross weight.  That's just the water vapor due to thrust to keep the airplane in level flight.  Climbing airplanes, which are heavier because they are at the start of the trip, require extra thrust to go uphill, so they'd put out lots more water vapor than descending airplanes at the end of the trip, which are usually operating near idle thrust.  How persistent a condensation trail is would depend on temperature and turbulence, but I don't know how.  

Also what effect does a head wind or tail wind have.

None.  The airplane moves relative to the air mass.  The contrail would move with the air mass.

[Howard Rush]

Also, newer airplanes burn less fuel than older airplanes, so I'd reckon that a new airplane would make less of a trail than an old one at the same flight condition.  There a lot of big airplane operating now, and they're bigger than ever, so I'd expect that to cause bigger trails. 

We looked at hydrogen-powered airplanes, a dumb idea that got more attention than it deserved.  I remember seeing a picture of a hydrogen-powered engine on a test flight.  It made a whopper contrail.

[Don Hutchinson AMA5402]

Are they not cruising at a higher altitude than they did years ago?? I seem to recall many flights to the Cape where they flew at ~ 30,000 ft. Now they seem to run around 38,000.

[Brian Hampton]

Gas turbines burn kerosene which, when combined with oxygen, forms CO2 and H2O. The water exits as a gas then as it cools in the ambient air it becomes visible as steam. That's why the vapour trail (contrail) seems to start somewhere around a plane's length behind the engine(s). On a clear, calm day (at altitude) it's not unusual to see a contrail running from horizon to horizon.

[Warren Walker]

I don't know, but I can cipher part of it.

Cargo and passenger versions operate at about the same gross weights, so which version shouldn't matter.  I figure that the amount of water vapor that an airplane in cruise leaves behind per foot traveled is proportional to the square root of its gross weight.  Its cruise speed is also proportional to the square root of gross weight.  That's just the water vapor due to thrust to keep the airplane in level flight.  Climbing airplanes, which are heavier because they are at the start of the trip, require extra thrust to go uphill, so they'd put out lots more water vapor than descending airplanes at the end of the trip, which are usually operating near idle thrust.  How persistent a condensation trail is would depend on temperature and turbulence, but I don't know how.  

None.  The airplane moves relative to the air mass.  The contrail would move with the air mass.

Thank you Howard, This morning when I was watching the planes fly by, I held my arm out, and measured the trails with my thumb and index finger,

they were only about one inch long. That's why I thought it was more about air temp combined with hot exhaust.

W.W.

[john e. holliday]

And some days you don't see a trail behind a plane.   It is the temp of the atmosphere at the altitude.  I've seen some trails start and stop, then start again.   And again it seems they last forever and other times disappear almost immediately.   How about those vapor trails when a fighter makes a high G turn.

[Phil Krankowski]

Altitude and weather affect these considerably.  They go from horizon to horizon some days (At least here in Ohio).  I have been told those are strategic bombers traveling halfway around the world...  I don't know for sure though.  I can say that the planes forming those trails cannot be identified with the naked eye, and a common pair of binoculars isn't very helpful either.

Phil

[dale gleason]

If the lapse rate is to the left of the dry adiabat, conditions are stable, assuming no turbulence from air mass conditions. In this case, vapour trails can remain visible for pretty long periods of time, when the water vapor freezes, not uncommon since its usually -54 degrees C or F near the tropopause, vapour trails can form manmade  "sun dogs".

However, if the lapse rate is to the right of the dry adiabat, instability is the result, thunderstorms, turbulence, and all kinds of things cause vapour trails to disseminate quickly. Hence, no "sun dogs"....

Copilots in the day had to know this kind of stuff, Captains didn't. Capt. Fancher can probably add pertinent facts such as climbing ladders with buckets of steam, Morse Code and Main Junction Boxes.

Merry Christmas, Everyone!

[Brett Buck]

We looked at hydrogen-powered airplanes, a dumb idea that got more attention than it deserved.  I remember seeing a picture of a hydrogen-powered engine on a test flight.  It made a whopper contrail.

   It's so dumb, my predecessors at work *gave back a bunch of money* after the Air Force and others wanted to build a hydrogen-powered spy plane, and Kelly Johnson told them it wouldn't work.

      Brett

[EddyR]

Ty is correct about WW2 The trails were so thick you could not see through them for days sometimes. The second wave  would fly above the trails to hide from the ground until they got near the target then drop below the trails to see the target. Some times they had to wait for days so they would clear out.
Ed

[Matt Colan]

CHEMTRAILS ARE 100% NOT REAL! The reason they show up is the airplane is flying in an area in the atmosphere that allows for water vapor to condense, and the airplane's exhaust is the Cloud Condensation Nuclei (CCN) that allows it to happen. Basically what you are seeing is man made clouds, and I think there is a name for the cloud now.

[Randy Cuberly]

Phtttttt! To you Roselind. AFAIC this is a huge pile of horse dung.  

In case she doesn't know it CA is in a massive drought! That's why the trees are dying...........there's no water to absorb. (or wasn't before the latest storms)
My hedge was just about gone (I watered it just enough to keep it alive) but now it is looking (almost) normal again.

There's an old saying "Don't believe anything you hear and only half of what you see".

That's more true now days than ever before AFAIC.

Politics suck, Jerry

Only one thing comes to mind....Archie Bunker and "Dingbat"  "once a Dingbat, always a Dingbat".

Randy Cuberly

[Steve Thornton]

The Boeing 707, 727, and Douglas DC8 era jets were rarely flying above 35,000 and usually 31,000 to 33,000.  Today a 737, 767, or AirBus will routinely fly at 41,000 and burn 50% less fuel.  The jet-stream is associated with the tropopause, which is the boundary between the stratosphere and the troposphere, and normally starts around 36,000 and is 4,000 to 6,000 thick.  Wind, temp, and pressure are what determine the density and dissipation of contrails.  If contrails do, in fact last longer today, that may have something to do with it....or it could be those tanks filled with thousands of gallons of globally warmed and fermented bovine flatulation that airlines spray from secret cargo compartments to create holes in the ozone...who really knows?
Steve

[Fredvon4]

I have been watching con trails as long as I can remember and asked my dad what they were probably when I was 5 or so. What he told me about how water vapor condenses in cold air and how combustion creates water out the exhaust. And soon after showed me the water coming out the exhaust of our 50s Ford station wagon early one morning as he was warming the car to go to work

Yeas later the conspiracy of chemical vapors to do nefarious stuff became news and such...I grinned at how little people actually know of common science. And I also though how illogical this grand conspiracy could be done with out some whistle blower providing proof.

I just had a look at flight radar, a free application

http://www.flightradar24.com/31.21,-98.01/5

There are 1500 aircraft over Texas as I type this. Ground air temp is 38F and clear dry air and not one contrail ( I have no idea what air temp or dryness is at flight level and too lazy to search it from my link to Gray Army Airfield WX

During Desert Storm we could not see the B52s but the con trail was distinctive most days as was the rolling thunder as they let loose the payloads.. grin

[Will Hinton]

If contrails do, in fact last longer today, that may have something to do with it....or it could be those tanks filled with thousands of gallons of globally warmed and fermented bovine flatulation that airlines spray from secret cargo compartments to create holes in the ozone...who really knows?
Steve

 

[Steve Fitton]

Also, newer airplanes burn less fuel than older airplanes, so I'd reckon that a new airplane would make less of a trail than an old one at the same flight condition.  There a lot of big airplane operating now, and they're bigger than ever, so I'd expect that to cause bigger trails. 

We looked at hydrogen-powered airplanes, a dumb idea that got more attention than it deserved.  I remember seeing a picture of a hydrogen-powered engine on a test flight.  It made a whopper contrail.

What was dumb (besides the difficulty of cryogenic fuel)? Is it the volume of the tanks required to carry enough LH2 vs much denser hydrocarbon fuel vs the size of the plane?

[Fredvon4]

speaking of dumb aircraft propulsion

"The first operation of a nuclear aircraft engine occurred on January 31, 1956 using a modified General Electric J47 turbojet engine.[1] The Aircraft Nuclear Propulsion program was terminated after the President's annual budget message to Congress in 1961"

"In May 1946, the United States Army Air Forces started the Nuclear Energy for the Propulsion of Aircraft (NEPA) project, which conducted studies until the Aircraft Nuclear Propulsion (ANP) program replaced NEPA in 1951. The ANP program included provisions for studying two different types of nuclear-powered jet engines: General Electric's Direct Air Cycle and Pratt & Whitney's Indirect Air Cycle. ANP planned for Convair to modify two B-36s under the MX-1589 project"

of course everyone know Wikipedia is terribly inaccurate

[Brett Buck]

What was dumb (besides the difficulty of cryogenic fuel)? Is it the volume of the tanks required to carry enough LH2 vs much denser hydrocarbon fuel vs the size of the plane?

     Essentially, yes. It's the same reason that it's a dumb idea for first stages of rockets. The advantage you gain from better ISP is offset by the size/mass of the tanks. They kept making Suntan larger and larger, and never got the required range. Look at an internal view of the shuttle tank- there's a teeny little oxygen tank at the top, on top of a gigantic hydrogen tank. It only worked at all because of the equally gigantic,  heavy, and extremely inefficient, solid boosters. Calculate the effective ISP of the shuttle stack including the solids, and it's lower than kerosene.

   Otherwise, LH2/Lox is pretty good. If nothing else, it's chemically pretty neutral. The engines tend to last and last. People tell me that they had dozens of missions worth of on-time on several different RL-10s, and they showed *no* detectable wear, same with the SSMEs and J-2. Chemical corrosion is the killer. That's why the Russians and their copper-lined steam engine tech engines leave a green streak in the exhaust. Copper transfers the heat out of the combustion chamber very well, but it also chemically corrodes. They make it thick enough to make the mission, but it's running on borrowed time.

     Brett

[Warren Walker]

    Essentially, yes. It's the same reason that it's a dumb idea for first stages of rockets. The advantage you gain from better ISP is offset by the size/mass of the tanks. They kept making Suntan larger and larger, and never got the required range. Look at an internal view of the shuttle tank- there's a teeny little oxygen tank at the top, on top of a gigantic hydrogen tank. It only worked at all because of the equally gigantic,  heavy, and extremely inefficient, solid boosters. Calculate the effective ISP of the shuttle stack including the solids, and it's lower than kerosene.

   Otherwise, LH2/Lox is pretty good. If nothing else, it's chemically pretty neutral. The engines tend to last and last. People tell me that they had dozens of missions worth of on-time on several different RL-10s, and they showed *no* detectable wear, same with the SSMEs and J-2. Chemical corrosion is the killer. That's why the Russians and their copper-lined steam engine tech engines leave a green streak in the exhaust. Copper transfers the heat out of the combustion chamber very well, but it also chemically corrodes. They make it thick enough to make the mission, but it's running on borrowed time.

     Brett




Brett, The knowledge you possess is mind boggling.

W.W.
     

[Howard Rush]

Those rocket scientists know their rocket science!

What was dumb (besides the difficulty of cryogenic fuel)? Is it the volume of the tanks required to carry enough LH2 vs much denser hydrocarbon fuel vs the size of the plane?

That's one thing.  a hydrogen-powered airliner would carry another fuselage or two just to hold the hydrogen.   The other dumb thing was getting the hydrogen.  Folks think of hydrogen power as an environmental free lunch.  We figured it would come from taking water apart with electricity.  Until the last coal-fired generating plant is replaced by a clean means of generating power, a hydrogen-powered airplane (or an electric car) is, for environmental purposes, powered by coal. 

[Brett Buck]

To give some reference, water is 62 lb per cubic foot, kerosene is 50 lb per cubic foot, and liquid hydrogen is ~4.4 lb per cubic foot. Kerosene and LOX has a maximum specific impulse of about 350, and Hydrogen and LOX is about 450-460. So it is 28% more efficient in terms of specific impulse, but requires 11x the volume compared to kerosene, so a given amount of "go", the LH2 tank has to be something like 8.6 times as big.

    It's even more exaggerated in air-breathing vehicles, since you get the oxygen from the air, and don't need an oxidizer tank. Then the entire system is driven by the fuel tank, which, as Howard notes, has to be gigantic, leading to more drag, leading to more thrust for a given speed, meaning higher fuel consumption, leading to a bigger tank, etc.

     Brett

[Brett Buck]

speaking of dumb aircraft propulsion

"The first operation of a nuclear aircraft engine occurred on January 31, 1956 using a modified General Electric J47 turbojet engine.[1] The Aircraft Nuclear Propulsion program was terminated after the President's annual budget message to Congress in 1961"

    Look up Project Pluto. It was essentially a nuclear-powered cruise missile with a nuclear ramjet. It could cruise around at low altitudes at Mach 2.5. They resolved the issue of the weight of the reactor shielding by not using any. It would toss a bomb, then just cruise around the USSR at low altitudes for weeks/months, destroying everything near the flight path with a combination of shock waves and radiation. Eventually it would either run down due to reactor poisoning, or some failure, then crash and spew the core around the crash site, irradiating it.

   It got as far as some tests of the engine on the ground, successful for the few minutes they could keep the necessary ram air flowing to it.

      Brett

[Bill Johnson]

    Look up Project Pluto. It was essentially a nuclear-powered cruise missile with a nuclear ramjet. It could cruise around at low altitudes at Mach 2.5. They resolved the issue of the weight of the reactor shielding by not using any. It would toss a bomb, then just cruise around the USSR at low altitudes for weeks/months, destroying everything near the flight path with a combination of shock waves and radiation. Eventually it would either run down due to reactor poisoning, or some failure, then crash and spew the core around the crash site, irradiating it.

   It got as far as some tests of the engine on the ground, successful for the few minutes they could keep the necessary ram air flowing to it.

      Brett

Now that's some scary stuff. Abandoned because if we built one, the Russians would build one, and there was no known defense. 

[Brett Buck]

Now that's some scary stuff. Abandoned because if we built one, the Russians would build one, and there was no known defense. 

   On the other hand, if your existence is sufficiently threatened, the only morality is winning.

     Brett

[Brett Buck]

Brett, The knowledge you possess is mind boggling.

W.W.
     

   Thanks, Warren. I picked some stuff up over the years.

    Brett

[M Spencer]

They must be geniuses ! 

[Bill Johnson]

   On the other hand, if your existence is sufficiently threatened, the only morality is winning.

     Brett

I was discussing Project Pluto with my daughter. Apparently, because of the high specific impulse, nuclear engines are again being considered. With some degree of shielding, the exposure to radiation from the critical mass would be more then offset by the reduction in exposure to radiation in space. Sufficient velocity can obtained to reach a destination, such as Mars, within days. 

[Brett Buck]

I was discussing Project Pluto with my daughter. Apparently, because of the high specific impulse, nuclear engines are again being considered. With some degree of shielding, the exposure to radiation from the critical mass would be more then offset by the reduction in exposure to radiation in space. Sufficient velocity can obtained to reach a destination, such as Mars, within days. 

   For space applications," conventional" engines (like NERVA)  would be very good, and the technology is well in hand since the 60's. IT was planned for use in the mid-70's as the upper stages of the Saturn V for Mars missions and other high-performance requirements. And it's not like you are going to contaminate space, what with the billions of unshielded fusion reactors we already have. The big problem will be the nuclear chicken littles that will prevent you from launching it. They are still terrified of launching even RTGs, and this is an order of magnitude more risky. You only have to use a small shield between the spacecraft and the engine.

    The other, potentially far more capable, technology is something like ORION, which amounts to dropping mini-bombs out the back of the spacecraft and detonating them, once a second or so, and letting them push on a huge "pusher plate" with a shock absorber to isolate the crew compartment(s). The interesting thing is that idea is that the larger it is, the better it works. Some of the studies have spacecraft the size of a city block with hundreds of crew. That is far from being a practical, implementable, technology and poses legitimate concerns for where you launch such a thing. It quickly became clear that you couldn't launch it from the ground, for obvious reasons, so you are then looking at many conventional launches to assemble the thing in lunar orbit, or something like that.

     Brett

[Chris McMillin]

Sitting on my deck this morning having a cup of coffee, I saw many airliners off in the distance leaving con trails.

My question is, what is the most contributing factor as to how long they will be.

Will a heavily laden cargo 747 using more fuel than a passenger 747 leave a longer trail, or does it have more to

do with air temp?

Also what effect does a head wind or tail wind have.

W.W.

Hi Warren,
Air mass stability in a condition conducive to forming them. Smooth air, big air mass that's the same.
As for "chemtrails";
I figured that if the airline can't get the bags on in time, the fuel on in a timely manner, schedule enough pilots for the daily operation, etc. there is no way they can get the chemicals on board properly either. Plus, how do they hide those tanks and weight computations from the mechanics and pilots? We're a pretty curious bunch and highly critical of what's going on around the very airplane in which we're going to be strapping our butts.
BTW, the small size of the military inventory of airplanes compared to civilian gives no credence to some massive fleet of military airplanes flying around chemically altering our weather for nefarious reasons or manipulating our minds. We have the international political scene for the former and the national news services for the latter!
Chris...

[Randy Cuberly]

Hi Warren,
Air mass stability in a condition conducive to forming them. Smooth air, big air mass that's the same.
As for "chemtrails";
I figured that if the airline can't get the bags on in time, the fuel on in a timely manner, schedule enough pilots for the daily operation, etc. there is no way they can get the chemicals on board properly either. Plus, how do they hide those tanks and weight computations from the mechanics and pilots? We're a pretty curious bunch and highly critical of what's going on around the very airplane in which we're going to be strapping our butts.
BTW, the small size of the military inventory of airplanes compared to civilian gives no credence to some massive fleet of military airplanes flying around chemically altering our weather for nefarious reasons or manipulating our minds. We have the international political scene for the former and the national news services for the latter!
Chris...

 

Hi Chris this is so funny I just had to respond...Hope you're well, and Merry  Christmas!
Unfortunately We'll never convince some people!

Randy Cuberly

[Warren Walker]

Hi Chris, That was some funny s#@t.

It's been to long, you need to make it to a Thursday night.


W.W.

[Brian Hampton]

I have been told those are strategic bombers traveling halfway around the world...  I don't know for sure though.  I can say that the planes forming those trails cannot be identified with the naked eye, and a common pair of binoculars isn't very helpful either.

Maybe the skies in Australia are clearer than in America but I took this photo of a Qantus plane flying over my city with the camera obviously zoomed in. Qantus is easy to pick out by eye because of the red tail. Notice how the exhaust doesn't condense behind each engine until roughly a plane's length behind.

[Dave Edwards]

a hydrogen-powered airplane (or an electric car) is, for environmental purposes, powered by coal. [/quote

Only a few percent of the hydrogen produced in the world is from electrolysis.  95% is produced from natural gas via a process called steam methane reforming.  Howard's point is spot on but the energy source is a different fossil fuel.

[Phil Krankowski]

Maybe the skies in Australia are clearer than in America but I took this photo of a Qantus plane flying over my city with the camera obviously zoomed in. Qantus is easy to pick out by eye because of the red tail. Notice how the exhaust doesn't condense behind each engine until roughly a plane's length behind.

My understanding is the military flight pattern is considerably higher than the commercial flight pattern. 

Yes, there could be simply a large separation between the plane and the trail, and the plane is hidden in the sky, as the days I see contrails form with no visible plane are very clear blue sky days.

Phil

[Norm Faith Jr.]

Hi Brett, I had always kicked around a theory of how a nuclear powered aircraft would work. I used my theory to generate some discussion among my aviation students. Using Boyle's Law, a small core reactor would heat O2 contained in a vessel, expand and regulate its expulsion to turn a free turbine attached to a gearbox driving a propeller. Some of my students suggested using liquid instead of O2. We all concluded that a conventional propeller was most practical. We also all concluded there was one major hurdle...How to replenish the O2 or liquid? It was a very stimulating conversation to say the least...and kept a few of them awake.  
Norm

   Look up Project Pluto. It was essentially a nuclear-powered cruise missile with a nuclear ramjet. It could cruise around at low altitudes at Mach 2.5. They resolved the issue of the weight of the reactor shielding by not using any. It would toss a bomb, then just cruise around the USSR at low altitudes for weeks/months, destroying everything near the flight path with a combination of shock waves and radiation. Eventually it would either run down due to reactor poisoning, or some failure, then crash and spew the core around the crash site, irradiating it.

   It got as far as some tests of the engine on the ground, successful for the few minutes they could keep the necessary ram air flowing to it.

      Brett